It is difficult to acquire a clear image of a scene that includes a bright light source in or near the field of view the camera. Glare reduces contrast and causes image fog and large area ghosts. Glare is hard to avoid and disrupts every optical system, including the human eye. Glare can be due to Fresnel reflection at lens surfaces, and diffusion in lenses. However, the two are often indistinguishable in an image.
International Organization for Standardization (ISO) standard 9358, 1994, describes a procedure for measuring glare and defines a veiling glare index as a ratio of the luminance in the center of a black target to the luminance of the surrounding large area uniform illuminant.
McCann et al. measured glare in multi-exposure high-dynamic range (HDR) images, McCann et al., “Veiling glare: The dynamic range limit of HDR images,” Human Vision and Electronic Imaging XII, SPIE, vol. 6492, 2007. Bitlis et al. constructed a parametric model for glare effects in images, “Parametric point spread function modeling and reduction of stray light effects in digital still cameras,” Computational Imaging V, SPIE 6498, pp. 29-31, 2007. A 4D to 8D transport tensor between light source and sensor have been developed for relighting and view interpolation, Sen et al., “Dual photography,” ACM Trans. Graph. 24, pp. 745-755 2006, and Garg et al. “Symmetric photography: Exploiting data-sparseness in reflectance fields,” Rendering Techniques 2006, 17th Eurographics Workshop on Rendering, pp. 251-262, 2006. Those methods can potentially be used to characterize glare, but they do not reduce or decompose glare on the image sensor of the camera. To reduce glare, some methods post-process images that already contain glare via a deconvolution.
Lenses can be designed to reduce glare by coating and shaping the lenses. A 4% to 8% transmission loss due to reflection means that a five to ten element lens can lose half the incident light and significantly increase reflection glare. Anti-reflective coatings make use of the light-wave interference effect. Vacuum vapor deposition coats the lens with a ¼ wavelength thin film using a √{square root over (n)} refractive index substance, where n is the index of refraction. Multi-layered coating reduces reflection to 0.1%. However, this is insufficient to deal with light sources which are more than four orders of magnitude brighter than other scene elements. Ancillary optical elements such as filters also increase the possibility of flare effects. Digital camera sensors are more reflective than film.
Meniscus lenses, which have a curved profile can act as a spherical protective glass in front of the lens assembly, prevent unwanted focused reflection from the sensor. The curved profile defocus causes large area flare rather than ghosts. Lens makers use an electrostatic flocking process to directly apply an extremely fine pile to surfaces requiring an anti-reflection finish. The pile stands perpendicular to the wall surfaces acting as Venetian blinds. This is an effective technique for lenses with long barrel sections.
Structural techniques include light blocking grooves and knife edges in lenses to reduce the reflection surface area of lens ends. Hoods or other shading device are recommended for blocking undesired light outside the viewing area.